One's in-laws were related to them by marriage. Amy Pond realised to her horror that she was the Eleventh Doctor's mother-in-law, through River Song. (TV: The Wedding of River Song)

When Lady Jane Grey's mother forced her into marriage with Lord Guildford Dudley, that made the Duke of Northumberland her father-in-law. The Duke had her appointed to the throne to gain power for himself. (TV: Lost in Time)